In the case of recognizing a hand-written character "R" as shown in FIG. 1, the concavities C1 and C2 and the hole H will be the key points for recognition. It will be understood that the concavities, convexities and holes are important characteristics of a configuration. For extracting these characteristics, for example, a convex hull of a configuration is generated and the original configuration is subtracted from the convex hull. However, in the digital image, the convex hull is not always precisely generated. In the digital image, the convex hull is generated by connecting top points successively with lines and by filling the inside of the lines. However, the line in the digital image differs according to the algorithm of line drawing. Usually one dot deviation may easily happen because an analog line does not always pass through the center of a digital pixel. When a pixel is selected, as a pixel on the digital line, from outside the pixel of the analog convex hull, the configuration generated by the subtraction of the original image from the convex hull does not represent an identifying characteristic itself. Usually a digital line is drawn by a graphic processor, so the digital line is drawn according to the algorithm of the processor.
When the convex hull is not drawn precisely, the analysis after the above subtraction cannot be well performed.